1. Field of the Invention
The invention relates to a modulation technique, and more particularly, to a modulation device for generating an optical signal with quadruple frequency and the modulation method thereof.
2. Description of the Related Art
The generation and transmission of millimeter-wave signals over an optical fiber have been intensively investigated for various applications, such as broad-band wireless access networks, radio over fiber (RoF), phase-array antennas, optical sensors, antenna remoting, and radars. The advantages of using millimeter-wave technology lie in the almost unlimited bandwidth and very low propagation loss. With the accelerated development of wireless communications, efficient and cost effective methods of generating and transmitting millimeter-wave signals are of utmost importance.
However, the traditional generation of optical millimeter-wave signals remains a major challenge in various optical fiber-supported systems. Limited by the frequency response of the regular modulator, and the electrical components and equipment, such as amplifiers, mixers, and synthesizers, are very expensive. Therefore, the cost effective generation of millimeter-wave signals is of great interest for various applications. Numerous optical millimeter-wave signals generation schemes based on frequency multiplication using modulators combined with optical filtering have recently been demonstrated. Nonetheless, these proposed systems are either very complex, suffer from low conversion efficiency and low signal-noise-ratio (SNR) or need more than one optical filter to remove undesired optical sidebands. The unavoidable optical filtering severely hinders the implementation of wavelength-division-multiplexer (WDM) RoF systems.
Therefore, to solve the above-mentioned problems, the present invention proposes a novel modulation device and method thereof for generating the optical signal with quadruple frequency. The present invention requires only a single integrated modulator to generate the optical signal, which simplifies the structure of circuit and dramatically reduces the total manufacturing cost.